Since the world energy crisis, it has become essential to be able to recover the maximum amount of hydrocarbons contained in underground formations.
Oil recovery using conventional techniques is limited to 33% on average. In order to increase this ratio and thus hope to make a petroleum reservoir profitable, enhanced recovery methods are implemented. These complex methods mainly consist in injecting specific fluids or heat.
Among the conventional recovery techniques, the most commonly used method consists in injecting, via an injection well, an aqueous fluid (generally water or brine). This fluid sweeps the underground formation so as to drive the hydrocarbons out of the pores of the rock where it is absorbed. Production wells allow a production effluent comprising a mixture of water, salts and hydrocarbons to be recovered.
There are several enhanced oil recovery methods. When compounds are added to the aqueous fluid injected, also referred to as sweep fluid, the method is referred to as tertiary chemical enhanced recovery. These chemical compounds are polymers, surfactants, alkaline compounds, or mixtures of such compounds. In relation to simple water or brine injection, the interest of the presence of a polymer is to increase the viscosity of the sweep fluid and therefore to improve the mobility ratio between the fluid injected and the hydrocarbons in place in the underground formation. The hydrocarbon recovery ratio is increased as a result of higher petroleum formation sweep efficiency. The polymers used in this method are generally polymers of high molecular mass used for their viscosifying properties.
Injecting polymers into the reservoir, which is commonly referred to as polymer flooding, is one of the most commonly used methods. These hydrosoluble polymers increase the viscosity of the water injected into the formation, thus modifying the oil/water mobility ratio. This favours a “piston” type sweep and results in a higher volumetric drainage efficiency of the oil in place.
The polymers are mixed at the surface and injected into the injection wells. They consist of structures of high molecular mass, typically above 106 g/mol. This fundamental characteristic is at the origin of their viscosifying power, but it also explains their main drawback, i.e. their mechanical degradation.
During the injection of polymers into the reservoir, the fluids displaced are in fact subjected to high shears and elongational flows, notably in the initial injection unit, the nozzles and the pumps, constrictions in the reservoirs and around the wells thus leading to partial degradation of the polymers and to an inherent limitation of their efficiency.
A solution to this problem has been unexpectedly found by limiting the mechanical degradation of the polymer solutions commonly used in EOR methods, by adding a small amount (of the order of some ppm by weight in relation to the aqueous phase) of a second variety of polymers having either a higher molecular mass or more fragile bonds. These additives, also referred to as sacrificial agents or sacrificial compounds in the description below, are preferentially degraded, dissipating enough energy to protect the “active” polymers.